1. Field of the Invention
The present invention relates to a motor control unit, such as an inverter or a servo amplifier, comprising a main semiconductor circuit for controlling a motor and a control circuit for controlling the operation of the main semiconductor circuit.
2. Description of the Background Art
Motor control units are becoming smaller because power switching elements, such as power transistors, are made more compact and are used on highly integrated power printed circuit boards. However, compact and highly integrated boards in small control units pose a problem with the dissipation of heat generated by the power components and other elements in the unit. In addition, as motor control becomes higher in level, the circuits become more complicated. For example, a large number of motors are often employed and they are frequently controlled by a single central controller.
FIG. 4A is a block diagram showing the circuit arrangement of a servo amplifier known in the art and FIG. 4B a circuit diagram of its main circuit. In the Figures, there is a power supply 1 and a converter circuit 21 comprising a diode module for converting the alternating current of the power supply 1 of a commercial frequency into a direct current. A capacitor 23 is used for smoothing the direct current converted from the alternating current by the diodes 21. An inverter circuit 22, comprising switching elements such as a transistor module, is used for converting the direct current smoothed by the capacitor 23 into an alternating current of selectable voltage and frequency by means of a PWM generator circuit 11 and supplying it to a servo motor 6. An encoder 7 is for detecting the position and velocity of the servo motor 6 and transmitting a signal to a position/velocity detector circuit 12. An operation circuit 13 compares an external position command from a controller 15 with a signal from the position/velocity detector circuit 12 and transmits a voltage command to the PWM generator circuit 11 according to the result of this operation. Current detector 16 is operative to detect an output current from the inverter circuit 22 and transmit that signal to the operation circuit 13. In FIG. 4A, the combination of these elements comprises a "motor control unit" 8 (a servo amplifier in the case of servo control). The operation of the servo amplifier for the servo motor is omitted here since it is well known.
An example of such motor control unit 8 is shown in a sectional view in FIG. 5 and a side view in FIG. 6. These two views show a motor control circuit from which a unit case and printed circuit boards have been removed. In these drawings, 2A indicates a unit case and 3A is a heat sink located at the back of the unit case 2A and has fins 3A-1 arranged outside the unit case 2A. The unit case 2A accommodates the components illustrated in FIGS. 4A and 4B including diode module 21, capacitor 23, transistor module 22, a main circuit printed circuit board 4 connected with the main circuit components, and a control circuit printed circuit board 5 comprising at least the operation circuit 13 and PWM generator circuit 11. The modules 21 and 22 have terminals 21-1 and 22-1 for electrical connection. The modules also have a contact section 2A-2 with heat radiating installation surfaces 21-2 and 22-2 for heat radiation from the front side and a heat sink section 2A-3 on the rear side. The terminals 21-1, 22-1 are connected to the printed circuit board 4 and the heat radiating installation surfaces 21-2, 22-2 are in contact with a connection surface 3A-2 of the heat sink 3A.
In this type of motor control unit 8, its unit case is generally contained in a control box and its heat sink is disposed outside the control box for enhanced heat dissipation.
FIG. 7 shows another arrangement example of the motor control unit 8, wherein fins 3B-1 of a heat sink 3B are arranged in parallel with the front face of the motor control unit 8 and its connection 3B-2 is mounted with the main circuit parts which generate much heat.
In Japanese Patent Disclosure No. 59895 of 1989, as shown in FIGS. 8A and 8B, an electronic component packaging module 40 is presented which transfers heat generated by electronic circuit packages 42 to a heat interface 47 located at the outside of an electronic circuit module 46 via a metal plate 44. A printed circuit board 43 is loaded with the electronic circuit packages 42, each incorporating a high-density electric circuit in a highly thermal conductive package case. The metal plate 44 connects to a heat pipe 48, which may otherwise be connected to or comprise an aluminum plate 49. At the other end of the packaging module 40 is an electronic interface 41 to which the printed circuit board components are electrically connected.
In the example shown in FIGS. 8A and 8B, the metal plate is in contact with the case of the electronic circuit packages, i.e. heat generator, and is heated thereby. Then, the plate conducts the heat to the heat pipe, and finally transfers it to the outside. This extended path of heat transfer results in low transfer efficiency.
Returning again to the conventional designs in FIGS. 5 and 6, the main circuit components, e.g. diode and transistor modules that generate much heat are generally rectangular and are provided with terminals for electrical connection on their wider-area side and with installation surfaces for heat dissipation on the other side. In the conventional design shown in FIG. 5 and FIG. 6, the wider-area side of the main circuit components is disposed in parallel with the front face of the motor control unit. This causes the width of the unit case (lateral direction in FIG. 6) to be restricted by the wider-area surface of the main circuit components, precluding design opportunities to make the unit case thinner. A wide installation area is therefore required when a plurality of motor control units are installed side by side, forming a string of control boxes.
Also, in the example shown in FIG. 7, the heat sink in this conventional design is rotated 90.degree. and is required to support a number of main circuit components that generate a substantial amount of heat. However, this design requires the fin height of the heat sink to be reduced by the thickness of the main circuit components, thereby restricting the amount of heat generated by the main circuit components that can be removed effectively.